Short circuit protection is required in a discharge lamp inverter application for safety and reliability reasons. When a shorted lamp condition occurs, a protection circuit is needed to reduce the power level or shut down the circuit completely to avoid circuit breakdown or other possible catastrophic situations.
FIG. 1 shows a typical CCFL inverter where the lamp voltage can be as high as one thousand volts. For human safety, UL60950 standard requires that the current through a 2 KOhm resistor should be within the following range when any two points in the inverter board is shorted by the resistor. 2 KOhm is a typical resistance of a human body.
      i          2      ⁢      k        ≤      {                                                      2              ⁢                                                          ⁢              mA                        ,                          when              ⁢                                                          ⁢              current              ⁢                                                          ⁢              is              ⁢                                                          ⁢              DC                        ,                                                                          0.              ⁢                                                          ⁢              7              ⁢                                                          ⁢              mA              ⁢                                                          ⁢              peak                        ,                                          when                ⁢                                                                  ⁢                frequency                            ≤                              1                ⁢                                                                  ⁢                KHz                                      ,                                                                                          0.7                                  *                                                                                                    ⁢                              (                KHz                )                            ⁢              mA              ⁢                                                          ⁢              peak                        ,                                          when                ⁢                                                                  ⁢                1                ⁢                KHz                            <              frequency              <                              100                ⁢                                                                  ⁢                KHz                                      ,                                                                          70              ⁢                                                          ⁢              mA              ⁢                                                          ⁢              peak                        ,                                          when                ⁢                                                                  ⁢                frequency                            ≥                              100                ⁢                                                                  ⁢                KHz                                      ,                              
FIG. 2 shows a prior art short-circuit protection method by sensing the inverter transformer's secondary winding current. An RC network, Rx and Cx, is added in series with the transformer's secondary winding to ground for sensing the transformer's secondary winding current. If the voltage drop of the RC network is larger than a threshold value, the short circuit protection is triggered. However, the RC network cannot pick up shorted current information when the transformer's secondary winding is shorted, such as at nodes Z and X. Another conventional method for short-circuit protection is to sense the duty cycle of the inverter. When the duty cycle is saturated and reaches its maximum value, the short-circuit protection is triggered. However, this method does not provide any direct information on the short-circuit condition.
An improved method is desired to detect a short-circuit condition even when the transformer's secondary winding is shorted and to trigger the short-circuit protection.